Sheet media delivery apparatus are used to deliver print media to printers. These apparatus are often incorporated into auxiliary bin sheet feeders, which are typically used to increase the media delivery capacity of nominally low-capacity inkjet or laser printers. They also are commonly used as part of an overall media transport system in large-capacity office printers and copier systems.
Various auxiliary bin sheet feeders were developed for many of the early laser printers, such as the Hewlett-Packard "LASERJET" II and "LASERJET" III printers. Since these printers were fairly large (in comparison to today's laser printers), the overall size of the auxiliary bin sheet feeders could also be fairly large without appearing out of place or taking up excessive room on a desk or printer stand that was sized to accommodate the printer.
Today's contemporary laser printers and inkjet printers are much smaller than their predecessors. However, the demand for increased media-handling capacities remains as strong as ever. In parallel with this reduction in printer size, office and consumer markets have also demanded a reduction in the size of the auxiliary bin sheet feeders that work with the newer printers.
A reduction in overall size of an auxiliary bin sheet feeder requires a reduction in the media delivery system. The media delivery system typically performs two functions. First, a sheet of media must be "picked" off of a stack of media. It is desired to pick a single sheet of media at time, which is known as "singulation." The second function of the media delivery system is to advance the picked sheet of media into a printer media feed system.
A common problem generally encountered in media delivery systems is "multipicking." Multi-picking may occur when underlying sheets in the stack are partially "dragged" out of the tray by the picking of sheets above. Many conventional media delivery systems are designed to provide minimal drag on the media sheet after it has entered the feed mechanism of the printer. While minimizing drag improves media alignment and positional accuracy in the printer, retracting the separator pad can create another type of multi-picking known as a "trailing pick." A trailing pick is caused when an underlying sheet is dragged by a sheet above it (generally the picked sheet) as the picked sheet is transported through the media delivery system.
A common technique for performing the pick operation employs a D-shaped wheel (D-wheel) that is rotated to cause a media sheet pick action. As the D-wheel is rotated, its curved portion contacts the media, urging it forward. As the D-wheel is further rotated it falls out of contact with the media at its flattened portion, allowing an upstream media transport mechanism to advance the media without resistance from the D-wheel. This arrangement is satisfactory so long as the media sheet is not bent around the D-wheel shaft during a feed operation. This situation may occur when the media tray is positioned at an angle relative to the feed mechanism. If the media sheet presses against the D-shaped wheel, significant drag on the media sheet results. Therefore, D-wheel systems are generally impractical for use in feed systems that require the media to be bent as it is fed to the printer, a common condition when auxiliary bin feeders are used with today's smaller printers.
The use of a D-wheel is also impractical for use in a low-profile auxiliary feed system applications that require the sheet media to be advanced a fair distance prior to entering the printer feed mechanism. For instance, a typical feed system may employ a D-wheel with a nominal diameter of about 2 inches (50 mm). Since about 3/4 of the perimeter of the D-wheel contacts the media during a feed operation, a D-wheel of this size can advance the media about 5 inches. In comparison, the D-wheel diameter of a low-profile auxiliary feed mechanism may be limited to about half this size. Such a reduced-size wheel can only advance the media about 2.5 inches, which is insufficient in most applications.
Another typical pick and feed arrangement is shown in FIGS. 1-3, which illustrate a pick roller system employed in a media sheet feed mechanism manufactured by the Epson Corporation. As shown in FIG. 1, the system comprises a pick roller 216 that is driven by a drive gear 210 mounted on a shaft 212, which in turn is coupled to a drive motor (not shown). A pivot arm 214 is mounted for rotation about shaft 212 and encloses a rubber pick roller 216. A driven gear 218 mates with drive gear 210, is rigidly connected to pick roller 216, and is mounted for rotation on a shaft 220. A spring washer 222 is positioned between an inner surface of arm 214 and driven gear 218, and performs a friction clutch function.
The pick and feed system is positioned above a media tray including a pressure plate 224 which supports a stack of media sheets 226. The tray is biased by a spring 228 into contact with the pick roller 216. An edge separator 230 is positioned to maintain an uppermost sheet on stack 226 in place until operation of the rubber pick roller 216.
The pick operation is illustrated in FIGS. 2 and 3. To implement a pick operation, drive gear 210 is driven in a counterclockwise (CCW) direction, thereby causing driven gear 218 to rotate in a clockwise (CW) direction. Due to the friction exerted by spring washer 222, arm 214 and pick roller 216 are caused to rotate in a CCW direction until arm 214 hits stop 232. This action causes pick roller 216 to come into contact with a top sheet 234 of stack 226. The top sheet is forced against pick roller 216 through the action of spring 228 on tray 224. Continued clockwise rotation of the pick roller 216 feeds the top sheet 234 from the stack 226.
As shown in FIG. 3, when the top sheet 234 is grabbed by a pair of feed rollers 236, the direction of rotation of the driven gear 210 is reversed to a CW direction, thereby causing the arm 214 and pick roller 216 to rotate in a CCW direction and out of engagement with the top sheet. The CCW rotation of pick roller 216 is required as the clutching action of spring washer 222 otherwise would cause pick roller 216 to impede the feeding of the top sheet. The CCW rotation of the arm 214 and pick roller 216 continues until the arm 214 hits a second stop 238.
The prior art device of FIGS. 1-3 is generally too large to be used in a low-profile auxiliary bin sheet feeder. In order to meet the height restrictions necessitated by the lower profile, the size of the pick roller and drive gear must be reduced, which adversely impacts the pick and feed performance. Furthermore, the positioning of the feed rollers adds extra length to the overall size of the feed system.
Reduced-size media delivery systems present other problems that are not generally encountered with larger systems. One such problem is that the size (diameter) of the drive motor on these systems may be limited. A drive motor with a 50% reduction in diameter may have 25% of the torque of a comparable full-size motor. As a result, the torque available to drive the system may be dramatically reduced.
In addition to the foregoing problems, the media feed system needs to be able to handle media that is not completely flat. Humidity will often cause media sheets to become corrugated in that the sheets have ripples or waves formed in them and are no longer flat. Corrugated sheets pose a problem for conventional feed systems because they have a tendency to jam, rip or become skewed when they enter a feed-roller assembly subsequent to being picked.
Therefore a need exists for an improved reduced-size auxiliary bin feeder and associated media delivery system. It is further desired to have a media delivery system that reduces multi-picks and trailing picks, and reduces the adverse effect of wavy media sheets. It is additionally desired to provide a media delivery system that requires less motor torque.